This invention relates to a wireless base station array antenna system in CDMA wireless communications. More particularly, the invention relates to an array antenna system for generating multiple beam signals split into an angle for each path of multipaths by an array antenna and beam former and combining the beam signals via a finger unit (despreader/delay-time adjustment unit) for each path to thereby demodulate received data.
Digital cellular wireless communication systems using DS-CDMA (Direct Sequence Code Division Multiple Access) technology have been developed as next-generation mobile communications systems for implementing wireless multimedia communication. In such CDMA communications, transmission information from a plurality of channels or users is multiplexed by spreading codes that differ from one another and transmitted via a transmission path such as a wireless link.
In wireless communications, radio waves from a transmitter arrive at a receiver via several paths (multipaths) having different path lengths. The receiver combines the radio waves. However, the combining of the radio waves is not performed by coherent addition, as a result of which fading occurs. Various diversity schemes have been proposed to deal with such fading. One example is a Rake receiving scheme. Rake reception is a technique which involves identifying signals that have passed through multipaths and combining the signals (by maximum-ratio combining) upon weighting them for reliability, thereby improving the characteristic. A receiver employing such Rake reception in CDMA communication has been proposed as a Rake receiver. FIG. 19A is a block diagram showing the construction of the prior-art Rake receiver, and FIG. 19B is a diagram useful in describing the delay profile thereof.
Shown in FIG. 19A are a searcher 1, fingers 21-23 each of which is provided for a path of multipaths, a Rake receiver antenna 3, a Rake combiner 4 for combining the outputs of the fingers, and a decision unit 5 for deciding the xe2x80x9c1xe2x80x9ds and xe2x80x9c0xe2x80x9ds of received data based upon the output of the combiner 4.
As shown in FIG. 19B, the reception level of the signal sent from a transmitter varies in the receiver in dependence upon each path of the multipaths, and the times of arrival at the receiver differ as well. The searcher 1 (1) measures the profile of the antenna reception level (the temporal transition characteristic of the level), (2) detects the multipaths from multipath signals MP1, MP2, MP3, which are higher than a threshold level, by referring to the profile, (3) identifies delay times from signal occurrence times t1, t2, t3 of respective paths of the multipaths or from a reference time, and (4) inputs despreading start timings s1-S3 and delay-time adjustment data d1-d3 to the fingers 21, 22, 23, respectively, that correspond to the respective paths.
The searcher 1 includes a matched filter 1a, which outputs the autocorrelation of a desired signal contained in the received signal. FIG. 19A illustrates the construction of one channel of a base station. That is, the reception output of the antenna 3 contains other channel components as well. The matched filter 1a uses the spreading code of its own channel to extract the signal component of its own channel from the antenna reception signal. The extracted signal component is delivered as the output. More specifically, when a direct sequence signal (DS signal) that has experienced multipath effects enters the matched filter 1a, the latter outputs a pulse train having a plurality of peaks conforming to arrival times and signal strengths and stores the pulse train in a RAM 1c via a low-pass filter 1b. A path detector 1d refers to the profile (FIG. 19B) that has been stored in the RAM 1c to detect each path constituting the multipaths as well as the delay times, and inputs the start signals s1-s3, which indicate the timings (chip synchronization timings) of the start of despreading, as well as the delay time adjustment data d1-d3, to the respective fingers 21, 22, 23 corresponding to the paths.
The fingers 21, 22, 23 corresponding to the respective paths are identically constructed and each includes a spreading code generator 2a for generating the spreading code assigned to its own channel, a multiplier 2b for multiplying the antenna reception signal by the spreading code to thereby despread the signal, a dump integrator 2c for performing dump integration, a delay time adjustment unit 2d for subjecting the despread signal to a time delay adjustment that conforms to the path, an arithmetic unit 2e which performs an operation for channel estimation, and a multiplier 2f for multiplying the input to the arithmetic unit 2e by the complex conjugate of the output thereof to estimate the channel and output a desired signal wave component corresponding to the channel. The complex conjugate is obtained by reversing the sign of the imaginary portion of the complex number. If the complex number is I+jQ, then the complex conjugate thereof is Ixe2x88x92jQ.
FIG. 20 is a diagram useful in describing the channel estimation operation. Shown in FIG. 20 is a transmitting antenna 3xe2x80x2 of a mobile station, the antenna 3 of the base station, the arithmetic unit 2e that performs the operation for channel estimation of the finger, the multiplier 2f, and a complex conjugate arithmetic unit 2fxe2x80x2 for outputting the complex conjugate. Let""s represent a signal transmitted from the transmitting antenna 3xe2x80x2 to the destination of the base station,  the influence of the wireless path and r the reception output of the base station. The arithmetic unit 2e outputs the product rs* of the input signal r and desired signal s. Accordingly, the output of the arithmetic unit 2e is       rs    *    =            s      ⁢              xe2x80x83            ⁢      ξ      ⁢              xe2x80x83            ⁢              s        *              =                  ξ        ⁢                              "LeftBracketingBar"            s            "RightBracketingBar"                    2                    ∝      ξ      
If there is no fluctuation in amplitude, the output of the complex conjugate arithmetic unit 2fxe2x80x2 becomes *, and the output of the multiplier 2f becomes       r    ⁢          xe2x80x83        ⁢          ξ      *        =            s      ⁢              xe2x80x83            ⁢              ξξ        *              =                  s        ⁢                              "LeftBracketingBar"            ξ            "RightBracketingBar"                    2                    ∝      s      
In other words, if the amplitude does not fluctuate, the signal s that was transmitted to itself is obtained from the multiplier 2f. Accordingly, the arithmetic unit 2e and multiplier 2f in FIG. 19A estimate and output the signal component of their own channel.
Thus, the fingers 21-23 corresponding to the respective multipaths despread the corresponding multipath signals MP1-MP3 by multiplying them by the spreading codes allocated to the channels and adjust the delays of the despread signals by the path delay times to make the timings agree. The Rake combiner 4 performs maximum-ratio combining of the finger outputs, and the decision unit 5 decides the received data based upon the output of the combiner.
Base station antennas of DS-CDMA communications system currently employ sector antennas. As shown in FIG. 21A, the 360xc2x0 perimeter of a base station is equally divided to split a cell into a plurality of sectors SC. A sector antenna is an antenna is that allocated to each sector SC. Since there is no directionality within a sector, the antenna is susceptible to interference from other users. Such interference from other users is the main cause of a decline in channel capacity and transmission quality. Research and development in regard to multiple-beam antennas and adaptive array antennas is being carried out in an effort to discover techniques for reducing such interference and improving transmission quality. If the multiple-beam approach is adopted, a directivity pattern is produced, as shown in FIG. 21B, to reduce the susceptibility to interference from other users and improve transmission quality.
As shown in FIG. 22, a multiple-beam antenna performs reception using an array antenna AAT consisting of a plurality of element antennas AT1-ATN, and applies beam forming to antenna output signals by means of a beam former BMF to electrically form multiple beams B1-BM of prescribed directivity. Each beam of the multiple-beam antenna possesses a directivity pattern of the kind shown in FIG. 23. Accordingly, radio waves emitted from an ith user (mobile station) residing in the directivity direction of beam 2, for example, are received by the array antenna AAT and the beam former BMF outputs the beams B1-BM. The power of beam B2, however, is greater than that of the other beams B1, B3-BM. Data is subsequently demodulated by performing despreading using the beam B2. Thus, in accordance with a multiple-beam antenna, reception is performed upon selecting the optimum beam on a per-user (channel) basis, whereby there are obtained such effects as a reduction in interference between channels, an improvement in reception SN ratio owing to a higher antenna gain and a reduction in terminal transmission power.
FIG. 24 is a block diagram showing the receiving section of a wireless base station. This is the circuitry for one channel. The apparatus includes a receiving array antenna AAT, which has a plurality of antenna elements AT1-ATN, receiving circuits RVC1-RVCN for performing high-frequency amplification, frequency conversion and quadrature detection, etc., of the received signals, and a reception beam former BF for electrically forming M-number of upward reception beams B1-BM by applying reception beam forming to the signals received by N-number of antenna elements AT1-ATN.
The receiving Section includes despreading circuits RSS1-RSSN the inputs to which are the N upward reception beams B1-BN, respectively, output by the reception beam former BF. The despreading circuits RSS1-RSSN apply despreading processing to the beams B1-BN, respectively, using despreading codes of the channel allocated to the user of interest and output despread signals (I, Q signals). A selection controller SCNT calculates the power of each despread signal and decides the beam for which power is maximum. A selector SEL selects the despread signal of maximum power and outputs the selected signal to a receiving unit RV. The receiving unit RV includes a synchronous detector SDM to which despread signals (I, Q signals) are applied for performing synchronous detection, and an error corrector ECC for applying error correction processing to demodulated reception data. The synchronous detector SDM detects a pilot signal, obtains the phase difference between this received pilot signal and an already known pilot signal and restores the phases of the despread I, Q signals by the amount of this phase difference.
As illustrated in FIG. 25, the beam former BF multiplies output signals x1-xN of the respective antenna elements by weights Wk,i to thereby implement phase rotation, and sums the products to electrically form M-number of upward reception beams 1-M each having a prescribed directivity. If x1(nTc) represents the reception signals of N-number of antenna elements and Wk,i represents the conversion coefficient of the beam former, then a signal yi(nTc) of an ith beam (i=1-M) will be expressed by the following:
yi(nTc)=xcexa3Wk,ixc2x7xk(nTc)(k=1-N)xe2x80x83xe2x80x83(1)
The directivity direction of each of the M beams can be applied to the array antenna by deciding the conversion coefficient Wk,i. As a result, a transmission signal from a user (mobile station) in a prescribed ith directivity direction can be obtained from the ith terminal of the beam former BF. FIG. 26 shows an example of a beam former which performs the operation of Equation (1) using an FFT for beam forming.
The N-number of antenna elements AT1-ATN (FIG. 24) input reception signals xi (nTc) (i=1-N) conforming to the received radio waves to the receiving circuits RVC1-RVCN. Each receiving circuit performs high-frequency amplification, frequency conversion, quadrature detection (QPSK detection) and A/D conversion of the input signal and inputs the resulting signal to the reception beam former BF. The reception beam former BF then digitally forms M-number of beams by applying beam forming to the N-number of input signals. That is, the reception beam former BF obtains the signal yi(nTc) of each of the beams 1-M through the conversion expressed by Equation (1). Next, the despreading circuits RSS1-RSSN perform despreading on a per-channel basis in regard to the plurality of beams, the selector SEL selects the despread signal for which signal power after despreading is maximum, and the receiving unit RV identifies the received data using the despread signal of maximum power.
Thus, the Rake receiver gathers together signals that have been scattered in time by multipaths and implements diversity reception to improve characteristics. Further, the multiple-beam antenna technique adopts the multiple-beam approach within a sector to reduce interference between channels, improve transmission quality and increase channel capacity.
In the prior art, however, the Rake receiver and multiple-beam antenna receiver are utilized separately of each other and, hence, there is a limit upon the improvement in transmission quality and reception characteristic.
Accordingly, an object of the present invention is to provide a wireless base station array antenna system in which Rake reception and a multiple-beam antenna scheme are combined to improve the transmission quality and the reception characteristic.
Another object of the present invention is to provide a wireless base station array antenna system for producing a plurality of beam signals separated into an angle for each path of multipaths, combining despread signals of one or a plurality of beams, the desired signal components of which are large, from all beams of all paths, and deciding the received data, thereby improving the transmission quality and the reception characteristic.
Still another object of the present invention is to provide a wireless base station array antenna system for producing a plurality of beam signals separated into an angle for each path of multipaths, generating despread signals of one or a plurality of beams, the desired signal components of which are large, from all beams of all paths, and combining the despread signals upon weighting each despread signal based upon adaptive control, thereby improving the transmission quality and the reception characteristic.
In accordance with the present invention, the foregoing objects are attained by providing an array antenna system of a wireless base station comprising (1) a beam former for forming a plurality of electric beams by applying beam forming to signals received by a plurality of antenna elements of an array antenna; (2) a despreading/delay-adjusting unit provided for each path of multipaths for despreading each of the plurality of beams conforming to signals which arrive via said path, applying a delay adjustment conforming to the path to despread signals having a desired signal component that is large, and outputting the result; and (3) a combiner for outputs from each of the despreading/delay-adjusting units by maximum-ratio combining. In accordance with this array antenna system, it is possible to construct a Rake receiver comprising a combination of Rake reception and a multiple-beam antenna scheme, as a result of which transmission quality and reception characteristic can be improved.
The array antenna system further includes a searcher for measuring time intervals at which each multipath signal occurs, and inputting despread start timing and a delay time signal to the despreading/delay-adjusting unit provided for each path of the multipaths. Providing the searcher makes it possible to readily control start timing of despread processing and delay time adjustment of each path.
The searcher measures and preserves delay profiles indicating temporal transitions of levels of all beams output by the beam former, and detects, on a per-beam basis, a path for which the beam level is high from the delay profile of each beam. The despreading/delay-adjusting unit (finger) provided for each path outputs a despread signal conforming to the beam of the high level from among the beams of its own path, and the combiner combines the despread signals output by each of the fingers and decides the received data. If this arrangement is adopted, the received data is decided upon combining the despread signals of beams which include more of the desired signal components. This makes it possible to improve transmission quality and the reception characteristic. In this case the searcher measures and preserves the delay profile of each beam by time sharing processing. This makes it possible to simplify the arrangement.
The array antenna system of the present invention further includes means for selecting the despread signals of one or a plurality of beams, for which power, or correlation value or SIR (signal/interference ratio) is large, from among all beams of all paths. The received data is decided upon combining the selected despread signals. If this expedient is adopted, despread signals are not selected and combined for each path. Rather, the received data is decided upon combining the despread signals of beams for which the power, correlation value or SIR is actually large, i.e., the beams which actually include more of the desired signal components, from among all beams of all paths. This makes it possible to improve transmission quality as well as the reception characteristic.
The array antenna system of the present invention further includes means for obtaining, from among all beams of all paths of the multipaths, a beam for which the result of measuring power, correlation value or SIR is largest, and selecting from each path a despread signal that conforms to this beam. The received data is decided upon combining the selected despread signals. This arrangement assures that even if beam measurement accuracy declines owing to noise, Rake combining will not be performed upon selecting a beam erroneously. As a result, transmission quality and reception characteristic can be improved.
Further, the array antenna system of the present invention has a space diversity configuration in which a plurality of branches each having a finger (despreading/delay-adjusting unit) for each path are arranged spatially at different directivities. From among despread signals of all beams that have entered the finger of each branch, those having desired signal components that are large are selected and combined. Since this arrangement provides the additional effect of space diversity, it is possible to achieve a further improvement in transmission quality and reception characteristic.
The array antenna system according to the present invention further includes (1) a received-data decision unit for deciding received data based upon the output of the combiner, (2) a selection unit for selecting, from all beams of all paths, or on a per-path basis, a plurality of despread signals having a desired signal component that is large, (3) an adaptive controller for deciding a weighting coefficient by adaptive control using the selected despread signals and results of deciding the received data, and (4) a weighting unit for multiplying each despread signal by the weighting coefficient and outputting the product. The combiner combines the weighted outputs and the received-data decision unit decides the received data based upon the output of the combiner. Adopting this arrangement makes it possible to construct a Rake receiver comprising a combination of Rake reception, a multiple-beam antenna scheme and an adaptive array antenna scheme. The result is an improvement in transmission quality and reception characteristic.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings.